1. Field of the Invention
The present invention relates to two-dimensional chromatography. Specifically this invention relates to devices and method for performing two-dimensional liquid or gas chromatography with partial modulation.
2. Description of the Related Art
Two-dimensional chromatography is known in the art. The fields of both gas and liquid chromatography utilize two-dimensional separation techniques to analyze sample matrices contained in a sample analyte.
Comprehensive two-dimensional liquid or gas chromatography utilizes two orthogonal columns connected in series to separate compounds within a sample. The term “orthogonal” as used herein means that the columns separate compounds within a sample based on two different properties of the compounds. Ideally, the different properties are independent of each other, resulting in a lack of correlation between the retention time of the compound in the first column and its retention time in the second column. The stationary phase in the second column and column sizeshould yield a faster separation than that of the first column. In the prior art, a sample is injected into the injection port and integrated with the mobile phase. The mobile phase is necessary to introduce and transport the sample through the column. In gas chromatography, the mobile phase is generally an inert gas and is often referred to as the carrier gas. In liquid chromatography, the mobile phase is a liquid of low viscosity and is often referred to as the carrier fluid. Injection of the sample may be by a syringe or operation of a valve or valves connected to a source or loop, among other methods. As the mobile phase transports the sample through the first column, the compounds in the sample are separated based on a first property. In the prior art, the first-column separated sample-bearing carrier exits the first column and is thereafter trapped and held by operation of a modulator, which releases the first column separated sample-bearing carrier in “plugs” or “packets” to the second column. The period during which a packet of first column separated sample-bearing carrier is held and released to enter the second column is typically measured in seconds and is referred to as the modulation period, which is equivalent to the maximum permissible secondary retention time. A detector at the exit of the second column measures the intensity of compounds in each packet at the conclusion of the second separation.
Primary retention time and secondary retention time identify each compound in the sample-bearing carrier in three dimensions, the third dimension being signal intensity identified by the associated detector. Modeling of the data with intensity in three-dimensions displays quantitative and qualitative properties of the compounds within the sample.
In prior art two-dimensional gas chromatography, the sample-bearing carrier was fully modulated.
Comprehensive 2-D chromatography with full modulation of the sample-bearing carrier is not without drawbacks as to temperature, size and power requirements of equipment, and time required for secondary dimension analysis. Each separation system, including the column and associated entry and exit apparati as well as the means for introducing the sample is operated throughout a column temperature range, which may be beyond the range of the modulator or the modulator value. In such cases, the sample-bearing carrier is removed from the higher-temperature environment of the first separation system to a second environment where full modulation occurs, then introduced to a second, potentially higher, temperature environment to pass through the second separation system, including the second column and associated entry and exit apparati. Such cooling and heating may alter the compounds within the sample-bearing carrier, skewing the results. Such a full modulator may require cryogenic cooling, restricting the size and portability. Finally, the secondary retention time period for full modulation should be long enough for a full separation in secondary dimension.
Partial modulation is available as an alternative to full modulation, but is not without its drawbacks. Selection of the secondary retention time period applicable is complicated by the structure and/or boiling point of the compounds with the sample-bearing carrier. The period of one complete cycle, the secondary retention time period, if held constant may truncate results of compounds which are more complex or which possess a higher boiling point, and therefore would appear at a greater secondary retention time and/or may result in low resolution of compounds which are less complex or which possess a lower boiling point and which therefore would appear at a lesser secondary retention time. While a shorter secondary retention time period is desirable for compounds which are less complex and/or which have a lower boiling temperature, a longer secondary retention time period is desireable for compounds which are more complex and/or which have a higher boiling temperature. A longer secondary retention time period results in loss of resolution when analyzing compounds which are less complex and/or which have a lower boiling temperature. The peaks associated with compounds which are less complex and/or which have a lower boiling temperature are not as well defined for analysis which a longer secondary retention time period is used. A shorter secondary retention time period results in loss of data when analyzing compounds which are more complex and/or which have a higher boiling temperature. A greater number of peaks are associated with compounds which are more complex or which are more complex are greater in number. A shorter secondary retention time period may result in truncation of data associated with one compound, thereby including the remaining data in the secondary retention time period associated with a later primary retention time. As the various compounds are separated in the first column, such shifting of data results in the incorrect assembly of the actual data and therefore misidentification of the actual compounds present.
It would be an improvement to the art to be able to sufficiently modulate an analyte-bearing sample to permit comprehensive 2-D liquid or gas chromatography that would not alter the chemical properties of primary separation, which would be capable of utilizing multiple secondary retention times periods.